Retroreflective sheets are widely used mainly for traffic safety purposes such as traffic signs, guide signs, warning signs, restriction signs. The retroreflective sheet includes plural retroreflective elements, a resin support sheet, a transparent cover film disposed on a surface side of the resin support sheet and a pressure-sensitive adhesive layer on a rear face side of the resin support sheet. The retroreflective element is supported by at least one of the resin support sheet and the cover film, and the resin support sheet and the cover film are connected to each other by heat press emboss forming from the rear face of the resin support sheet so as to form a connection part. A groove of the connection part is formed on the rear face side of the resin support sheet. A release film is further laminated to the retroreflective sheet. For the release film, a release material, such as a polyester film, a polyethylene laminated paper, polypropylene and polyethylene, coated with a release agent such as a silicon compound and a fluorine compound, may be used. For a pressure-sensitive adhesive for forming the pressure-sensitive adhesive layer, a solvent-type pressure-sensitive adhesive, an emulsion-type pressure-sensitive adhesive, and a pressure-sensitive adhesives that can be cured by electron beams or ultraviolet rays, or the like may be used, for example. Such a retroreflective sheet is adhered to a substrate such as a metal substrate like aluminum, a steel plate, a coated steel plate and a stainless steel plate; a plastic substrate like a fiber reinforced plastic (FRP) and a rigid vinyl chloride; and a composite substrate including aluminum plates sandwiching polyethylene or the like, by using an adhesion device or an adhesion machine such as a squeegee, a hand roll and a mangle roll, so as to be used as a display board, a signboard or the like (see JP3278299 and JP2000-329918A, the contents of which are incorporated by reference).
However, when adhering the retroreflective sheet to the substrate, unless air trapped between the substrate and the retroreflective sheet is expelled, the air is enclosed between the substrate and the retroreflective sheet. In this case, when a temperature of the substrate increases due to sunlight or the like, the enclosed air expands, and accordingly a trouble such as blisters, bubbles, wrinkles and exfoliations may occur to the retroreflective sheet. In order to prevent this, when adhering the retroreflective sheet to the substrate, it is necessary to apply some pressure so as to expel the air trapped between the substrate and the retroreflective sheet. In addition, in the case where the retroreflective sheet in a distorted state is adhered to the substrate, problems such as blisters, bubbles, wrinkles, exfoliations or the like occur over the course of time due to a residual stress inside the retroreflective sheet.